This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast which protects a fluorescent lamp from dissipating excessive power at or near the end of the life of the lamp.
A fluorescent lamp is an evacuated glass tube with a small amount of mercury in the tube. The tube is lined with an adherent layer of a mixture of phosphors. Some of the mercury vaporizes at the low pressure within the tube and a filament or cathode in each end of the tube is heated to emit electrons into the tube, ionizing the gas. A high voltage between the filaments causes the mercury ions to conduct current, producing a glow discharge which emits ultraviolet light. The ultraviolet light is absorbed by the phosphors and re-emitted as visible light.
A gas discharge lamp is a non-linear load, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp conducts. Once the lamp conducts, current through the lamp will increase rapidly unless there is a ballast in series with the lamp to limit current.
A magnetic ballast is an inductor in series with a lamp for limiting current. An electronic ballast is a power supply especially designed for gas discharge lamps and typically includes a rectifier for changing alternating current (AC) into direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 khz. Some electronic ballasts include a boost circuit between the rectifier and the inverter for increasing the voltage supplied to the inverter.
It is conventional in electronic ballasts for gas discharge lamps to provide protection for the ballast or for a person in the event of one or another fault condition. For example, U.S. Pat. No. 5,099,407 (Thorne) describes a ballast including a "runaway protection circuit" to prevent the ballast from destroying itself when the lamp is removed while power is applied. U.S. Pat. No. 5,101,140 (Lesea) describes an electronic ballast including a series capacitor for limiting output current in the event of a short circuit. U.S. Pat. No. 4,893,059 (Nilssen) describes a ballast that protects a person from "through lamp leakage" when the person removes only one end of a tubular lamp from its socket and touches the exposed pins. The leakage is detected and the ballast shuts off before the person is electrocuted.
The fluorescent lamp has been made very much more efficient in recent years by reducing the diameter of the tube and by operating the lamp at higher temperatures. Fluorescent lamps are designated by a code in which the diameter of the tube is expressed in eighths of an inch. Thus, "T12" refers to an older, tubular lamp having a diameter of one and one-half inches. The newer, more efficient T8 lamps are tubular and one inch in diameter. T5 fluorescent lamps are now being introduced and there are laboratory prototypes of T2 lamps. Some smaller diameter lamps are folded to make a less elongated light source. A folded lamp is known as a compact and is typically a T4 lamp.
A smaller diameter fluorescent lamp typically runs at high bulb temperature, e.g. 200.degree. F. near the filaments. At the end of the life of such a lamp, one filament usually stops emitting electrons before the other filament and the lamp begins to rectify the current through it. This is called diode mode operation. If a ballast having a capacitive current limiter powers the lamp, the current through the lamp is forced to remain balanced in each direction but the voltage across the lamp becomes asymmetrical, i.e. there is a net DC potential across the lamp. When a lamp operates in diode mode, there is a large voltage drop inside the glow discharge adjacent the failed filament. Ions in the discharge are accelerated to high energies and bombard the filament, dissipating large amounts of energy and raising the already high temperature of the filament even further.
Occasionally, a filament will become so hot that the glass tube melts and the lamp implodes, producing anything from cracked glass and melted plastic to a shower of droplets of molten glass and hot glass splinters. A fire may be ignited. Such failures were almost unknown with T12 or T8 lamps because the large diameter of the tube provided clearance between the filament and the tube wall. T2, T4, and T5 lamps have such little clearance that additional heating of the filaments from operating in diode mode can readily cause an implosion.
Diode mode of operation can often damage a ballast because of the asymmetrical current drawn from the ballast and because of the high voltages the ballast is called upon to produce. It is known in the art to detect diode mode for the purpose of protecting the ballast, e.g. U.S. Pat. No. 5,394,062 (Minarczyk). The ballast described in the Minarczyk patent only detects excess voltage across the lamp, i.e. the ballast detects voltage magnitude and not direction, while it is necessary to detect and react to excessive AC voltage across a lamp, the sensitivity of the small diameter lamps is so great that it is also desired to detect voltage asymmetry of no more than 20 volts DC in a lamp that is operating at 120 volts AC. By detecting diode mode, a ballast can be shut down well before overheating of the filaments can occur.
There are several technical problems with incorporating lamp protection circuitry into an electronic ballast. One problem is that large voltages, often with momentary asymmetry, are applied to a lamp in order to initiate conduction through the lamp. For example, it may be necessary to apply 300 volts rms to ignite a 120 volt fluorescent lamp and yet it is desired to detect that the same lamp is operating at 220 volts rms. It is desirable that a ballast react to an excessive, steady state, AC voltage by shutting off and not react to an even larger, asymmetrical, transient voltage for starting the lamp.
A second problem is that the operating voltage of fluorescent lamps increases with age and that operation in diode mode is far more destructive than operating at slightly higher but symmetrical AC voltage. As used herein, "DC sensitivity" refers to operation in diode mode and "AC sensitivity" refers to operation with a symmetrical AC voltage across the lamp. Thus, the need is for a lamp protection circuit that does not shut off the lamp during starting and which has much higher DC sensitivity than AC sensitivity. It is desired for the protection circuitry to trigger at a DC offset of no more than 10 volts and at an AC voltage exceeding normal operating voltage by 100 volts.
In order to protect a lamp, or a ballast, or a person touching the lamp or the ballast, it is not necessary that the ballast be completely turned off. Some ballasts react to faults by literally shutting off some or most of the circuitry in the ballast. Other ballasts, e.g. ballasts having series resonant, parallel loaded outputs, increase the operating frequency of the ballast, thereby reducing the voltage applied to the lamp. The voltage is reduced to the point that the lamp stops conducting. As used herein, "shutting off" an inverter means, at a minimum, reducing the power supplied to a lamp in order to prevent harm to the ballast, the lamp, or a person coming into contact with the ballast or the lamp.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast including circuitry for protecting gas discharge lamps.
Another object of the invention is to provide an electronic ballast that can detect an asymmetry in the voltage across the lamp of as little as twenty volts and shut off the ballast.
A further object of the invention is to provide an electronic ballast that does not detect starting voltages as a fault condition.
Another object of the invention is to provide an electronic ballast that detects diode mode of operation and over-voltage.
A further object of the invention is to provide an electronic ballast that responds quickly to a fault condition to prevent destruction of a lamp powered by the ballast.
Another object of the invention is to provide an electronic ballast that includes relatively few additional components to provide protection for a lamp powered by the ballast.
A further object of the invention is to provide lamp protection circuitry with high DC sensitivity and low AC sensitivity.